The present invention relates to a separator for a fuel cell that enlarges the cross-sectional area of gas passages and improves the power generation efficiency of the fuel cell, and to a method for manufacturing the separator.
A solid polymer type fuel cell is formed by laminating a plurality of power generating cells, each of which includes a flat plate-like electrode structure (MEA: Membrane Electrode Assembly) and separators on both sides. The electrode structure has a three-layer structure. The electrode structure has a pair of gas diffusion electrodes forming a positive electrode (cathode) and a negative electrode (anode), and an electrolyte membrane between the gas diffusion electrodes. The electrolyte membrane is formed, for example, of an ion-exchange resin. The separators are laminated to contact the gas diffusion electrodes. Gas passages are formed between each separator and the corresponding gas diffusion electrode. In a fuel cell, oxidizing gas such as oxygen or air flows through the gas passages facing the gas diffusion electrode on the positive electrode side (positive electrode layer), and hydrogen gas as fuel flows through the gas passages facing the gas diffusion electrode on the negative electrode side (negative electrode layer). This causes electrochemical reaction to generate electricity.
Japanese Laid-Open Patent Publication No. 2004-265856 discloses a metal separator used in a fuel cell. As shown in FIG. 9, the separator has flat walls 35 and linear protrusions T, which are arranged alternately. Each protrusion T is formed by a pair of side walls 36 and a bottom wall 37. The side walls 36 are bent in relation to the flat wall 35. The inclination angle of each side wall 36 relative to the associated flat wall 35 is expressed by α. A gas passage forming groove 18 is formed between each pair of the protrusions T. Openings 19 of the gas passage forming grooves 18 are closed by a positive electrode layer 14, so that gas passages 20 to correspond to the flat walls 35.
The side walls 36 are inclined relative to the flat walls 35 in order to increase the contact area of the bottom walls 37 with the positive electrode layer 14, so that adequate energization and a sufficient cross-sectional area of the gas passages 20 are ensured. The side walls 36 are inclined through precision coining by a forming apparatus.
Japanese Laid-Open Patent Publication No. 2004-281146 also discloses a separator used in a fuel cell. As shown in FIG. 10, the separator 16 has a plurality of linearly extending protrusions T. A gas passage forming groove 18 is formed between each pair of the protrusions T. The protrusions T contact a positive electrode layer 14 (or a negative electrode layer), so that the passage forming grooves 18 form gas passages 20. Each protrusion T includes a pair of side walls (vertical walls) 36, a pair of overhangs 367, and a bottom wall 37. The side walls 36 extend substantially perpendicularly to the flat walls 35. Each overhang 367 obliquely projects from the upper end of the corresponding side wall 36. Each bottom wall 37 connects the upper ends of adjacent overhangs 367 to each other, and is parallel with the flat walls 35.
However, the separator 16 shown in FIG. 9 has the following drawbacks when being coined. That is, in this case, boundaries 39 between the flat walls 35 and the side walls 36 and boundaries 39 between the side walls 36 and the bottom walls 37 are deformed plastically. Therefore, when the inclination angle α of the side walls 36 relative to the flat walls 35 is less than or equal to 80°, stress concentrates in the boundaries 39. This is likely to cause cracks and fractures. It is therefore difficult to reduce the inclination angle α, while maintaining an adequate thickness of the separator 16 and ensuring sufficient cross-sectional area of the gas passages 20.
In contrast, the separator 16 shown in FIG. 10 can ensure a sufficient contact area of the bottom walls 37 with the positive electrode layer 14 using the overhangs 367. However, as disclosed in paragraphs 0041 to 0073 of the description and FIGS. 7 to 14 of the Japanese Laid-Open Patent Publication No. 2004-281146, the overhangs 367 require a number of steps to form. The formation of the overhangs 367 is significantly burdensome and can increase manufacturing costs.